It is well known that such documents as established by national administrations and other authorities must be as safe as possible against being tampered with in order to be quite certain of the identity of the bearer.
European published patent application No. EP-A-0 013 418 describes a method of manufacturing a card comprising two transparent outer protective layers, e.g. made of polyvinyl chloride, two inner layers, and a perforated or porous protected layer which is inserted between said inner layers and which allows the material of said inner layers to penetrate therein on melting, thereby making a seal.
U.S. Pat. No. 3,836,754 describes an identification card having a plurality of areas of different optical transmission coefficients at a given wavelength of light, e.g. holes filled with colored galatene.
U.S. Pat. No. 3,802,101 describes an encoded identity card including a sheet having a metal core which is pierced by openings which in turn are covered with transparent plastic sheets.
Proposals have already been made to prepare such documents in the following manner:
Identification information is applied by any suitable means (e.g. printing and/or transferring decals) onto a paper or card medium which may optionally be covered with a layer of polyethylene or gelatin, and on which a security background is preferably pre-printed. The resulting substrate is then covered on its front and back surfaces with one or more layers of un-peelable protection. In an identity card of known type, the paper is covered with a complex of polyester and polyethylene. The protective layers are caused to adhere to the substrate by heat sealing.
Under such conditions, forgers seeking to tamper with information recorded on a card merely soak the card in a bath of one or more known polyethylene solvents such as xylene, acetone, methylethylcetone, benzene alcohols, etc., thereby dissolving the polyethylene after a few hours, and giving access to the paper (on which traces of polyethylene may sometimes remain). Once accessible, the document is altered, and subsequently its protective layers are reconstituted in such a manner as to ensure that the document has the same general appearance as before.
Preferred implementations of the present invention mitigate the above drawback by showing up any attempt at tampering with the original information.